The conventional electric window blind is generally provided with batteries so as to enable the electric window blind to remain in an operating condition in case of a power outage. However, adding the batteries to the electric window blind is by no means a sure thing that the electric window blind will work at the time when the power outage takes place, in view of the fact that the batteries may have been badly corroded. In addition, the batteries must be replaced periodically with new ones, which result in an increase in the cost of the use of the electric window blind.